The elder brother of North Korea's new leader says bribery and corruption will be the undoing of a country ruled by an inexperienced young man, newly published emails show.

Kim Jong-Nam, the half brother of Kim Jong-Un, who took control of the hermit state after the death of their father late last year, said corruption was so rampant that the country's political system would not survive.

"The amount of bribery merchants have to offer to high-level officials in order to survive keeps rising," Jong-Nam told Japanese journalist Yoji Gomi by email.

"Such a corrupt system will inevitably collapse. It reminds me of the situation right before the USSR collapsed," he added in a message dated December 7.

The comments were published in Bungeishunju magazine and come just weeks after Gomi released a book of email exchanges and interviews with Jong-Nam, who he first met in Beijing in 2004.

"North Korean youngsters are influenced by the winds of South Korean culture and capitalism and live their own lives... seeking to dodge strict control.

So I advised Jong-Un to offer a more abundant life through reform and opening," he said in an August 5 email

In a message sent on December 13, six days before a tearful newsreader announced the death of Kim Jong-Il, Jong-Nam said a disastrous attempt to control prices and clamp down on the black market had been pivotal in the breakdown of societal controls.

"People's trust in the North's leadership has been broken due to the aftermath of the currency revaluation," he said, referring to a widely unpopular move to swap hyper-inflated notes.

"Ageing leader, inexperienced successor, tattered economy... It is seen that the political situation in the North is dangerous."

The currency revaluation in late 2009 ended in failure, and a senior official believed to be in charge of the project was reportedly executed. Jong-Nam said his younger brother was not really old enough to become leader of North Korea.

"I can't see on the kid's face (Jong-Un) any sense of duty or seriousness as the next leader of such a complicated country as North Korea, and any deep thoughts on the future of country," he said, in an email sent on November 4.

In his commentary on the emails, Gomi said Jong-Nam's criticism of the North became even harsher after the bombing of Yeonpyeong island in November 2010, which killed four South Koreans.

In the email sent on November 27, four days after the shelling attack, Jong-Nam said: "The North's military carried out the attack to prove the reason of their existence and their status and to legitimise the country's nuclear weapons program."

Referring to Jang Song Thaek, the husband of Jong-Il's younger sister, who is seen by many analysts as the power behind the throne, Jong-Nam said even he could not rein in the powerful armed forces.

"The father is old, the successor is young, the uncle has no military experience.

So there's practically no one who can control the military," he said. Gomi said he had originally chosen not to include these mails in his book because he was worried that they could be misinterpreted and could lead to tighter controls on the flow of information into North Korea.

But he said he had decided to publish them now because he believed he had a duty "to convey in an accurate and straightforward manner Jong Nam's true intentions at a time when the world's focus is on North Korea".